Prusa I3 Autobed leveling...defining the probe bed position...how does this work?

This is what I currently have set for this...but everytime I G29, the box it is making is in the center of the 8 x 8 platform, and try as I might, I cannot get this damn thing to extend to the edges.


// set the rectangle in which to probe
#define LEFT_PROBE_BED_POSITION 15
#define RIGHT_PROBE_BED_POSITION 170
#define BACK_PROBE_BED_POSITION 180
#define FRONT_PROBE_BED_POSITION 20

Can anyone explain how these numbers are supposed to correlate to the four edges of the print bed?
I've had it crash off the back multiple times...and I don't dare continue this trial and error until I understand how these numbers are supposed to work.

Thanks!

Remember your home position is 0,0 for your x,y axis. That means that your opposite corner is 200,200. Measure your offset for your probe to your print nozzle and calculate in millimeters where you want the probe to land. There is also a section you will have to enter the offset to get the printer to print.

Edited 1 time(s). Last edit at 01/01/2015 11:36PM by jrcaster.

Quote
jrcaster
Remember your home position is 0,0 for your x,y axis. That means that your opposite corner is 200,200. Measure your offset for your probe to your print nozzle and calculate in millimeters where you want the probe to land. There is also a section you will have to enter the offset to get the printer to print.

Yeah that's kinda what I thought, but going 155 on the back I'm already off the platform and I get a crash.

So something isn't right.

Anyone?